There are methods which, at the time of irradiating a recording medium in which a plurality of color developing layers of which threshold values of color developing temperature are different are laminated with laser light, form a full color image by varying strength of the laser light and an exposure time to the laser light in accordance with color to be developed.
Laser light with which a recording medium is irradiated is converted into heat, and the heat is propagated within the recording medium. The heat is propagated not only in a lamination direction in which color developing layers are to be laminated, but also in a direction orthogonal to the lamination direction. For this reason, as the heat is propagated from a laser spot of the laser light formed on the recording medium toward the inside of the recording medium, the heat is diffused in the direction orthogonal to the lamination direction. Accordingly, an area in the color developing layer where color is developed becomes larger than an area of the laser spot, and as a result a desired image cannot be formed.